The present invention is concerned with a component that functions with bulk acoustic waves, particularly a bandpass filter.
One filter technology that functions with acoustic waves is known as a thin film bulk acoustic wave resonator or FBAR, and another is a bulk acoustic wave filter, which is known as a BAW filter. Both of these filters can be implemented as bandpass filters by interconnecting different thin-film resonators (FBAR) built using FBAR technology.
BAW components have a multi-layer structure having a plurality of layers arranged one over the other. Here, it is possible to have a vertical structure consisting of a plurality of BAW resonators as is known and disclosed in U.S. Pat. No. 5,821,833, whose disclosure is incorporated herein by reference thereto.
FIG. 1A shows an exemplary BAW component having a resonator stack which includes a plurality of resonators arranged over one another.
A resonator that functions with bulk acoustic waves, for example a resonator R3A in FIG. 1A has a piezoelectric layer PS3, which is arranged between two electrode layers E5 and E6. It is known that instead of only one piezoelectric layer, a layer sequence can also be used. The layers are deposited on a substrate TS one after another and structured into the resonators. As illustrated, the stack includes electrode layers E1 and E2, which are on opposite sides of a piezoelectric layer PS1, and electrode layers E3 and E4, which are on opposite sides of a piezoelectric layer PS2. The electrode layers, such as E1, E2, E3 and E4, are structured in the lateral plane so that sub-electrodes E11 and E12 are formed from the layer E1; sub-electrodes E21 and E22 are formed from the layer E2 and sub-electrodes E31, E32, E41 and E42 are formed from the remaining two electrode layers. The sub-electrodes E11 and E21 lying over one another form together with the piezoelectric layer PS1 lying therebetween a sub-resonator R11. Sub-resonators R12, R21 and R22 are built in a similar manner. Between the electrode layers E2 and E3 and layers E4 and E5 are coupling layer systems KS1 and KS2, respectively, which systems are acoustically at least partially transmissive. These coupling layer systems along with sub-resonators, such as R21 and R22 arranged therebetween, guarantee an acoustic coupling in the vertical direction between the subresonators R11 and R12 in the vertical direction with the resonator R3A and the transmission of an electrical signal is possible from the sub-resonator R11 to the sub-resonator R12 in case of the galvanic isolation therebetween.
The sub-resonator R11 is connected to a first electrical gate P1 used to couple in an electrical signal. The sub-resonator R12 is connected to a second electrical gate P2 to couple out the electrical signal.
The electrical series connection of the acoustically coupled sub-resonators R11/R21 and R12/R22 of the two resonator stacks formed adjacent one another takes place by means of the continuously formed electrode layer E5.
The multi-layer structure shown in FIG. 1A in schematic cross-section is arranged on the substrate TS. It is known that a resonator stack can be arranged over a hollow space provided in the carrier substrate or over an acoustic mirror, such as AS.
It is known that the resonators and/or sub-resonators arranged over one another or adjacent to one another can be electrically connected to one another and, together, can provide a filter element or a filter circuit, particularly a bandpass filter. A bandpass filter of this sort can be used together with additional filters also in a duplexer.
In FIG. 1B, an exemplary known implementation of the electrical and acoustic connection via a middle electrode layer ME of two series resonators SR1 and SR2 arranged over one another is schematically illustrated in the left side of the Figure and are shown in a circuit diagram on the right side of the Figure.
In FIG. 1C, a circuit diagram on the left side of the Figure for a basic element of a ladder type filter with a series resonator SR arranged in a signal line and a parallel resonator PR parallel to a signal line is shown. On the right side of the Figure, the schematic design of such a basic element is shown in cross-section. The series resonator SR is arranged laterally adjacent to the parallel resonator PR.
In FIG. 1D, a T-element can be formed with: series resonators SR and SR1 arranged adjacent to one another and a parallel resonator PR. These are shown in a circuit diagram on the left side of the Figure and shown in cross-section on the right side of the Figure. Also known is a filter structure shown in FIG. 1E on the left side, which consists of a plurality of interconnected T-elements. This filter structure is suited particularly to transmission of symmetrical or balanced electrical signals. The series resonators SR11, SR12, SR13 and SR14 are arranged in a first signal line. The series resonators SR21, SR22, SR23 and SR24 are arranged in a second signal line. The two signal lines are connected to one another using parallel branches which include at least two parallel resonators PR1, PR2 and PR3, PR4. The resonators are arranged adjacent to one another and are illustrated in the left side of the Figure as a circuit diagram and have a schematic top view of the filter structure shown in the right side of FIG. 1E.
Series and/or parallel resonators can be connected, in each case, to an inductance, for example a bond wire, in series in order to increase the passband width. It is also possible in case of interconnection of a plurality of series resonators with a plurality of parallel branches to bridge some of the series resonators between the adjacent parallel branches or to omit parallel branches between two series resonators. Some of the resistors can be replaced, for example, with a capacitance, an inductance or an LC element. For subsequent adaptation of the static capacitance of a BAW resonator, for example improving the rejection band selectivity, a capacitor can be connected in parallel to it.
In a BAW resonator, preferably only one acoustic mode, which is a main mode, is excited which, however, is often coupled to additional, undesired, particularly lateral, acoustic modes. Due to this mode coupling, the emergence of the acoustic energy out of an active resonator region occurs, which leads to energy losses and, thus, to a high insertion loss in the signal to be transmitted. The localization of the acoustic wave in the active resonator region occurs, for example, through the attenuation of the excitation in the edge region of the BAW resonator. This can be attained through addition of an additional material frame in the edge region of the upper electrode of the resonator or also through a special electrode configuration with sides that do not run parallel to one another, as shown, for example, from the International Publication WO 01/06646, whose disclosure is incorporated herein by reference thereto.
In addition, it is known from International Publication WO 01/99276 A1 and U.S. Pat. No. 6,448,695 B2, which claims priority from the same British Application and whose disclosure is incorporated herein by reference thereto, that resonators that are arranged laterally adjacent to one another and electrically connected through a common electrode can be acoustically coupled additionally through a lateral acoustic mode. This additional acoustic signal path contributes, in this case, to a particularly efficient signal transmission between two resonators, and it is possible to attain a particularly low insertion loss in the signal.
Almost all of the previously known BAW resonators, particularly with ladder-type filter topologies, have in common that they do not satisfactorily fulfill the demanding requirements of mobile radio operators for rejection band selectivity. A problem which is not uncommon and which is difficult to solve in a BAW filter design is that of attaining a low insertion loss.